1. Field of the Invention
The present invention relates generally to apparatus and methods for taking core samples of subterranean formations. More specifically, the present invention relates to a core bit having features to control flow of drilling fluid into a narrow annulus between the core bit inside diameter and the outside diameter of an associated core shoe of a coring apparatus for reduction in drilling fluid contact with, and potential invasion and contamination of, a core being cut.
2. State of the Art
Formation coring is a well-known process in the oil and gas industry. In conventional coring operations, a core barrel assembly is used to cut a cylindrical core from the subterranean formation and to transport the core to the surface for analysis. Analysis of the core can reveal invaluable data concerning subsurface geological formations—including parameters such as permeability, porosity, and fluid saturation—that are useful in the exploration for petroleum, gas, and minerals. Such data may also be useful for construction site evaluation and in quarrying operations.
A conventional core barrel assembly typically includes an outer barrel having, at one end, a core bit adapted to cut the cylindrical core and to receive the core in a central opening, or throat. The opposing end of the outer barrel is attached to the end of a drill string, which conventionally comprises a plurality of tubular sections that extends to the surface. Located within, and releasably attached to, the outer barrel is an inner barrel assembly having an inner tube configured for retaining the core. The inner barrel assembly further includes a core shoe disposed at one end of the inner tube adjacent the throat of the core bit. The core shoe is configured to receive the core as it enters the throat and to guide the core into the inner tube. Both the inner tube and core shoe are suspended within the outer barrel and rotate freely with respect to the core bit and outer barrel. Thus, as the core is cut—by application of weight to the core bit through the outer barrel and drill string in conjunction with rotation of these components—the core will traverse the throat of the core bit to eventually reach the rotationally stationary core shoe, which accepts the core and guides it into the inner tube assembly where the core is retained until transported to the surface for examination.
Conventional core bits are generally comprised of a bit body having a face surface on one end. The opposing end of the core bit is configured, as by threads, for connection to the outer barrel. Located at the center of the face surface is the throat, which extends into a hollow cylindrical cavity formed in the bit body. The face surface includes a plurality of cutters arranged in a selected pattern. The pattern of cutters includes at least one outside gage cutter disposed near the periphery of the face surface that determines the diameter of the bore hole drilled in the formation. The pattern of cutters also includes at least one inside gage cutter disposed near the throat that determines the outside diameter of the core being cut.
During coring operations, a drilling fluid is usually circulated through the core barrel assembly to lubricate and cool the plurality of cutters disposed on the face surface of the core bit and to remove formation cuttings from the bit face surface to be transported upwardly to the surface through the annulus defined between the drill string and the wall of the bore hole. A typical drilling fluid, or drilling mud, may be a hydrocarbon or water base in which fine-grained mineral matter is suspended. The core bit includes one or more ports or nozzles positioned to deliver drilling fluid to the face surface. Generally, a port includes a port outlet at the face surface in fluid communication with a bore. The bore extends through the bit body and terminates at a port inlet. Each port inlet is in fluid communication with an upper annular region formed between the bit body and the inner tube and core shoe. Drilling fluid received from the drill string under pressure is circulated in the upper annular region, which enables the port inlet of each port to draw drilling fluid from the upper annular region. Drilling fluid then flows through each bore and discharges at its associated port outlet to lubricate and cool the plurality of cutters on the face surface and to remove formation cuttings as noted above.
In conventional core barrel assemblies, a narrow annulus exists in the region bounded by the inside diameter of the bit body and the outside diameter of the core shoe. The narrow annulus is essentially an extension of the upper annular region and, accordingly, the narrow annulus is in fluid communication with the upper annular region. Thus, in addition to flowing into the port inlets, the pressurized drilling fluid circulating in the upper annular region also flows into the narrow annulus between the bit body and core shoe. The drilling fluid bypassing the port inlets and continuing into the narrow annulus is commonly referred to as the “flow split.” The narrow annulus terminates at the entrance to the core shoe and any drilling fluid flowing within its boundaries is exhausted proximate the throat of the core bit. As a result, drilling fluid flowing from the narrow annulus, or flow split, will contact the exterior surface of the core being cut as the core traverses the throat and enters the core shoe.
A high flow split can create significant problems during coring operations, especially when coring in relatively soft to medium hard formations, or in unconsolidated formations. Drilling fluids discharged near the core as it traverses the throat and enters the core shoe can invade and contaminate the core itself. For soft or unconsolidated formations, these drilling fluids invading the core may wash away, or otherwise severely disturb, the material of the core. The core may be so badly damaged by the drilling fluid invasion that standard tests for permeability, porosity, and other characteristics produce unreliable results, or cannot be performed at all. Fluid invasion of unconsolidated or fragmented cores is a matter of great concern in the petroleum industry as many hydrocarbon-producing formations, such as sand and limestone, are of the unconsolidated type. For harder formations, drilling fluid coming into contact with the core may still penetrate the core, contaminating the core and making it difficult to obtain reliable test data. Thus, limiting fluid invasion of the core can greatly improve core quality and recoverability while yielding a more reliable characterization of the drilled formation.
Apparatus and coring methods for reducing fluid invasion of the core have been proposed. U.S. Pat. No. 4,981,183 to Tibbitts and U.S. Pat. No. 5,568,838 to Struthers et al. each disclose a flow restriction in the narrow annulus between the bit body inside diameter and the core shoe outside diameter. The flow restriction is comprised of a bearing surface on the core shoe in sliding contact with a mating shelf on the core bit. A substantial fluid seal is essentially formed in the contact region between the bearing surface and the shelf. However, maintenance of a reliable fluid seal during a coring operation requires continuous contact, or at least close proximity, between the bearing surface and the shelf as the bearing surface rotates relative to the shelf.
U.S. Pat. No. 5,460,230 to Dekoster also discloses the introduction of a flow restriction into the narrow annulus between the bit body inside diameter and the core shoe outside diameter. The fluid restriction is comprised of an annular lip formed at the end of the core shoe and a corresponding annular slot on the inside diameter of the core bit body. The substantially mating relationship between the annular lip and annular slot forms a 180 degree bend in the flow path of the narrow annulus. The dimensions of the annular lip and annular slot can be selected such that flow resistance is optimized and, accordingly, flow split is minimized. However, in addition to requiring the formation of an annular lip on the core shoe, the bit body and core shoe must be manufactured to precise tolerances in order to maintain the desired flow restriction while simultaneously minimizing interference contact between the core shoe and bit body as they rotate relative to one another during coring.
In addition to the flow restriction in the narrow annulus, the Dekoster patent discloses modifications to the ports extending through the bit body. Each port includes a deflected outlet nozzle and a bore inclined away from the cut core. The deflected outlet nozzle and inclined bore direct drilling fluid away from the core as the drilling fluid exits the port outlet on the face surface of the bit. To further encourage flow of drilling fluid away from the core, a fluid stream detachment effect is created by locating each port outlet on a lateral surface between two successive blades. The blades, which have cutters disposed thereon, are raised relative to the lateral surfaces and port outlets. The fluid stream detachment facilitates the flow of drilling fluid away from the core as the drilling fluid exhausts through the deflected outlet nozzles. Such modifications to the bore and port outlet of each port, however, do not affect flow split through the narrow annulus.
Therefore, a need exists in the art of formation coring for apparatus and methods of reducing the flow of drilling fluids in the narrow annulus between the bit body inside diameter and the core shoe outside diameter, or flow split. Minimization of flow split will reduce fluid invasion of the core. Conventional core bits have a highly robust design and construction as the mechanical and chemical environments in which the core bit must operate can be severe. Thus, any core bit having features for reducing flow split must be reliable and easily maintained while, at the same time, not compromising the ruggedness of the core bit during coring operations.